1. Technical Field
This invention relates to radio frequency ranging systems that provide an estimate of the distance between two radios and/or the relative or absolute position of a radio.
2. Description of Related Art
Radio-frequency (RF) ranging technology provides distance and relative position between objects without the need to take physical measurements. RF ranging systems calculate the distance between two objects based in part on the time a radio signal propagates between those objects. In air, radio signals propagate at a constant rate, roughly equal to the speed of light.
Knowing the absolute position of a sufficient number of objects (such as e.g., cell phone towers fixed relative to the earth), an RF ranging system can be used to determine the absolute position of other objects. An RF ranging system may utilize a signal from the Global Positioning System (GPS) to provide absolute location. In many situations, however, GPS signals are either unavailable or actively denied to a potential user. RF ranging systems can provide accurate information regarding distance and/or location even in locations or situations without access to GPS signals.
Two general types of RF time-of-flight ranging systems exist. The first type utilizes highly accurate, synchronized clocks to calculate the propagation time. For example, in such a system, a first radio and a second radio both contain clocks synchronized such that the second radio receives the signal from the first radio and compares the departure time to the arrival time. The system multiplies this propagation time by the speed of light to estimate the distance between the radios. Maintaining a system of clocks synchronized to the level of accuracy required to make this type of system practicable for ranging purposes represents a significant drawback.
The second type of RF time-of-flight ranging system does not require absolute clock synchronization. Typically, these systems compute clock offsets or correction factors to account for clock references that are not synchronized. The clock offset can be computed as an unknown along with the coordinates of a positioning system, such as GPS, based on a plurality of observations. For time-of-flight measurements between two radios, or where the above method otherwise fails, the clock offset may be observed and computed by monitoring the phasing of the received signal, relative to the local clock.
RF ranging systems may utilize a round-trip time-of-flight measurement to compute the distance between the radios. These types of systems can be further classified into “round-trip full-duplex” configurations and “round-trip half-duplex” configurations. In a round-trip full-duplex configuration, a first radio transmits a signal to a second radio, which then retransmits the same signal back to the first radio without performing any calculations using the signal. After receiving the retransmitted signal from the second radio, the first radio compares the departure time to the arrival time to calculate the round-trip signal propagation time. The system multiplies this time by the speed of light and divides by two to estimate the distance between the radios.
In a round-trip half-duplex configuration, a first radio transmits a signal to a second radio, which then performs calculations using that signal. The second radio then transmits a new signal, which often contains the results of the calculations performed by the second radio, back to the first radio. The first radio then utilizes the data from the second radio and other data within the first radio to calculate the round-trip signal propagation time. The system multiplies this time by the speed of light and divides by two to estimate the distance between the radios.
Early approaches to RF ranging systems were primarily dominated by continuous wave (CW) and other narrow bandwidth systems. While CW systems, such as tellurometers, enable long distance ranging in the tens of kilometers and accuracies of 1 cm at a distance of 1 km in low multipath environments, these systems often suffer from poor multipath performance, susceptibility to jamming, have a high probability of detection and interception, and tend to cause interference to other communications systems. In recent years, CW techniques have mostly been superseded by ultra-wideband (UWB) and various spread spectrum systems.
The most common approaches to improve RF time-of-flight ranging performance are to increase system power (signal-to-noise ratio) and to increase system bandwidth. Scaling bandwidth up to multi-GHz levels, or equivalently scaling to very short pulses in time can achieve centimeter-level position accuracies and resolution. When UWB systems are constrained to operate within FCC regulations however, their range is typically limited to a few hundred meters using directional antennas in low multipath environments, and typically much less than 100 meters indoors with omni-directional antennas. UWB systems also require complex RF electronics which can drive up system cost and risk.
Spread spectrum technologies have several advantages over UWB systems that include low probability of intercept, low susceptibility to jamming, and increased range over UWB systems when radios are required to conform to FCC regulations. Many commercial spread spectrum systems operate as FCC unlicensed radios in the industrial, scientific, and medical (ISM) bands of 915 MHz, 2.4 GHz, and/or 5.8 GHz. Some commercially available implementations, such as Zigbee radios, include RF ranging with accuracies of several meters and rely on time difference of arrival and received signal strength measurements. Cellular phone networks also estimate radio locations through angle of arrival and received signal strength measurements with accuracies in the tens of meters. In addition, in dense signal environments, certain infrastructure radio signals such as television, radio, and Wi-Fi hotspots can be used to estimate location within several meters, depending on the availability of signals.
Accurate RF ranging systems can be implemented using a common modulation technique known as direct-sequence spread spectrum (DSSS), in which the input data (often called the baseband signal) is multiplied by a predetermined repeating sequence of “1” and “−1” values (referred to as a pseudonoise or PN code sequence). Each value in a PN code sequence is referred to as a chip. The PN code sequence is at a higher frequency than the baseband signal and thereby “spreads” the baseband signal to a wider band or “spectrum.” The resulting PN-coded signal is modulated to an RF carrier frequency and transmitted to a receiver. In the receiver, a correlator multiplies the PN-coded signal by the same PN code sequence (since 1×1=1 and −1×−1=1) to reconstruct the original baseband signal.
RF ranging systems using asynchronous clocks can use PN codes to measure small phase offsets between radios. Specifically, the autocorrelation function of a PN code sequence displays a sharp peak when the PN code sequences are aligned. The peak moves with the phase offset between the two PN code sequences. By accurately determining the phase offset, i.e, relative timing, between a transmitter and receiver, an RF ranging system can improve its ability to estimate distance.
The theoretical ranging accuracy limit for the variance for an RF time-of-flight ranging system using DSSS is given by the Cramer-Rao Lower Bound,
      σ          t      ^        2    =      1          8      ⁢                        π          2                ·        SNR        ·        α        ·        N        ·                  BW          2                    where SNR is the average signal-to-noise ratio of the two radios, α is the number of code copies averaged, BW is the occupied spectral bandwidth, N is the number of chips in each PN code sequence, and σ{circumflex over (t)}2 is the lower bound of the variance of the round-trip time of flight. The lower bound of the standard deviation of the one-way distance, σ{circumflex over (d)} is given by the equation
      σ          d      ^        =            σ              t        ^              ·          c      2        ·          k      medium      where c is the speed of light and kMedium is a correction factor for the speed of light through the intervening medium between the originator and transponder.
There is a need for a high-precision RF ranging system that obviates or at least mitigates one or more of the shortcomings of previous techniques to provide more accurate estimates of distance between two radios. Through the use of techniques discussed below, the present invention can provide an RF ranging system using DSSS that approaches the theoretical ranging accuracy limit.